This invention relates in general to lasers and, more particularly, to a laser apparatus including a telescoping movable lens structure which can be focussed to compensate for undesired thermal lensing effects.
It is generally desirable that the output beam of a laser exhibit controlled divergence and high quality in applications such as laser rangefinding, target designation, laser drilling, laser cutting, laser welding and the like. However, this is often difficult to achieve due to undesired effects related to the dissipation of power in the laser rod. For example, thermal focal lensing, thermally dependent rod end curvature and birefringence of the rod all affect the rod focal length. These power dependent effects cause the uncompensated output beam quality to be power dependent.
Lasers invariably include an optical element or material which is the lasing medium. This medium receives energy from an external source such as flash lamps, for example. Most of the energy applied to the medium is dissipated as heat to external cooling devices or fluids. This heat transfer results in thermal gradients within the lasing medium with higher temperatures at the center of the medium than at the exterior surfaces of the medium. These thermal gradients result in thermal lensing effects due to the combination of thermal expansion and stress birefringence. An example of this can be seen using a cylindrical rod-shaped lasing medium which is cooled on its outside diameter. In this case, the temperature distribution in the rod cross section is parabolic with a peak at the center dependent on power input and heat transfer coefficients. Thus, the higher thermal expansion at the rod centerline and the resulting shear stress manifested as stress birefringence cause the rod to behave as a positive lens.
If no compensation is made for the thermally induced lens or thermal lensing, the divergence of the laser output beam varies with input power and will also vary during the start-up period while the steady state gradient is being established. In other words, thermal lensing is power and time dependent. It is known in the art to place additional optical elements in the path of the laser beam to compensate for this undesired effect and therefore achieve increased collimation.
One approach which has been employed to compensate for laser thermal lensing is to position a two lens telescope in the laser beam path. The spacing between the two lenses is made to be dynamically variable. At start-up and as the output beam power and the power dissipated in the laser active element correspondingly vary, so too is the spacing between the two lenses varied to compensate for thermally induced variation in the focal length of the laser active element. The time programmed motion of the telescope and steady state positions at various power levels are controlled by external electronics which are programmed based on empirical and analytical data.
Prior U.S. patents which employ such a telescoping lens arrangement to compensate a laser for thermal lensing include U.S. Pat. No. 4,949,358 issued to Kantorski et al. and U.S. Pat. No. 5,001,718 issued to Burrows et al. Kantorski et al. employ both a telescope to compensate their laser for thermal lensing and a polarization rotator to ameliorate undesired birefringence effects. Burrows et al. employ a linear stepper motor in a feedback loop to adjust the distance between the lenses of their telescope to minimize thermal lensing.
U.S. Pat. No. 4,065,207 issued to Zavitsanos et al. discloses a programmable power attenuator for lasers wherein a motor drives a screw to change the position of an inner cylinder within an outer cylinder. As the inner cylinder changes position, the size of an absorptive cell within the attenuator correspondingly changes. The absorptive cell is filled with a gas which absorbs the energy of the laser beam passing therethrough. A variable length path is thus provided to the incident laser beam such that the intensity of the beam can be attenuated according to the particular path length which is selected using the stepper motor. The screw which drives the inner cylinder back and forth is separated from the inner and outer cylinders. The Zavitsanos et al. patent does not address the problem of laser thermal lensing but rather is directed toward laser power attenuation.